1. Field of the Invention
The present invention generally relates to integrated circuits (ICs) that have a hysteresis capability, and more specifically to a circuit for programming an IC's hysteresis level by adjusting a programmable hysteresis signal.
2. Description of the Related Art
It would be desirable to provide an IC such as a temperature controller, an A/D converter, a comparator or a controller with the capability of having externally programmable hysteresis levels. These types of ICs typically compare an input signal to a set point to generate an output signal. Hysteresis prevents the output signal from jittering when the input signal is hovering near the set point, and also holds the output signal for a longer period of time. The output signal may be used, for example, in a temperature controller to trigger a cooling fan, produce a warning signal or shut down a system. In this type of application it is clearly advantageous to have a signal that does not turn on and off rapidly. However, the amount of hysteresis required for different temperature sensing applications, for example, may vary substantially, with a typical range being 0.5.degree.-10.degree. C.
The TMP-01 programmable temperature controller by Analog Devices, Inc., the assignee of the present invention, described in U.S. Pat. No. 5,225,811, "Temperature Limit Circuit With Dual Hysteresis", provides over- and under-temperature signals that incorporate an adjustable level of hysteresis. In the TMP-01 both the high and low temperature set points and the hysteresis levels are set by a common impedance circuit. The impedance circuit is a voltage divider that is connected between an external reference voltage pin provided by the TMP-01 and ground. The set points are determined by the ratios of the resistors in the voltage divider, and the hysteresis current is set by the divider's total resistance.
By using a common circuit to establish both the set points and the hysteresis, the TMP-01 can be offered in an 8-pin package. However, in this configuration the values of the resistors in the voltage divider are a function of both the desired set points and the desired hysteresis level. Users have had trouble computing the resistor values to obtain both the correct set points and hysteresis. Furthermore, adjusting the set points affects the hysteresis and vice versa.
Another problem occurs when the reference voltage pin is used to bias another circuit. If the circuit loads the reference voltage pin, the hysteresis current will be affected. The loading effects can be reduced by using a high impedance buffer between the external pin and the circuit, but this adds components which occupy valuable board space.
A programmable hysteresis circuit that provides a number of predetermined hysteresis levels and easily computable intermediate levels that are not dependent on the set points or affected by loading the IC's reference voltage is needed.